The invention relates to a method for the mechanical removal of moisture from a filter cake, wherein a treatment fluid flows through the filter cake as a result of a difference in pressure, and to an apparatus for implementing the method.
In existing filtration by differential gas-pressure, shrinkage cracks often occur during the moisture-removal phase, thereby considerably impairing the result of filtration. As a result of the physical/chemical composition of solid and liquid, characteristic crack patterns often form on the surface.
These cracks, which in most cases extend through the entire filter cake as far as the filter cloth, mainly occur when processing fine-grain, especially mineral, solid materials, having a complex chemical structure. It is to be assumed that crack formation is governed by capillary forces, by an electrostatic interaction between particles, as well as by the pH value and by the nature and concentration of the ions in the liquid contained in the filter cake. Cracking behavior is also affected, for example, by the filtration pressure-difference, the height of the cake and by the filter cloth.
The occurrence of cracking has a negative effect on further treatment of the filter cake. Thus, both in vacuum filtration and in pressure filtration, a bypass flow of air takes place through the cracks produced. Cake washing deteriorates drastically, since the washing fluid prefers to pass through the cracks rather than through the accumulated material being washed.
To prevent crack formation in cake-forming filters, it is known that the filter cake can be compressed by the external application of compressive forces, thereby reclosing open cracks. Another known possibility is to carry out further filtration through cracks which have formed, but this causes rewetting of the filter cake which has already formed, and consequently causes fresh cracking.
A further possibility for preventing crack formation consists in adding small quantities of solid substances, such as cellulose or glass fibres, to the suspension being filtered, which, however, because of the cost involved or the negative effect on the purity required in the substance produced, can only rarely be implemented.
It is known from EP 0 179 446 B1 (corresponding to U.S. Pat. No. 4759858) that, in order to prevent crack formation, a filtering medium can be designed, in which, by providing non-filtering bars on the filtering medium, a division into filtration areas is achieved, with an associated division of the filter cake produced into a plurality of small sections.
It is the aim of this invention to create a method of the type mentioned earlier, which permits filtration with moisture removal and washing of the filter cake by a very low consumption of treatment fluid. A further aim is to present an apparatus for the implementation of this method.
Procedurally, the aim is achieved by the characterizing features presented in claim 1. In this method, the treatment fluid condenses in the filter cake, wherein the mother liquid originally present in the filter cake is displaced, and wherein at the same time further impurities are washed out of the filter cake. Replacement of the mother liquid by the treatment fluid ensures that no crack formation occurs.
A particular development, in which a piston-flow-like displacement of the mother liquid from the filter cake is executed, is advantageous. Ideally, a sharp displacement front forms between the mother liquid and the treatment fluid.
Repetition of this flow-through process after intermediate cooling of the filter cake increases the efficiency of the method, especially in respect of the washout effect.
If the treatment fluid has the properties of a low ionic strength, a low ion concentration and a mild, essentially neutral pH value, the mother liquid between the individual particles of the filter cake is largely replaced by a largely neutral fluid. The electrical interaction between the particles is thereby weakened, and a smaller distance between the solid bodies is made possible. The cake shrinks as moisture removal progresses. At a degree of saturation of approximately 80%, the tensile stresses reach a maximum, and crack formation occurs, if the liquid bridges between two or more particles can no longer absorb the forces.
The shrinkage of the filter cake, and thereby the tendency towards cracking during moisture removal, increase with the distance between the particles at the start of or during moisture removal. The reasons why the particles are not always in mutual contact when the cake is formed include electrostatic interaction between the particles and between the particles and mother liquid. These interactions increase in significance the smaller the particles are. They are also affected by the electric charge of the particles, by the ionic strength and the pH value of the liquid contained in the filter cake, so that replacement of mother liquid by a treatment fluid with the properties according to the invention, executed in the method according to the invention, produces a greater particle density and thereby a reduction of the tensile stresses between the particles, which reduces the risk of crack formation.
It is also an advantage for the treatment fluid, or the treatment liquid produced from it by condensation, to have a great capacity to absorb volatile or soluble substances from the filter cake. In this sense, solvents such as alcohol, acetone, sulphuric acid or soda lye can be used as the treatment fluid. In this way, such substances are washed out or extracted, without the need for prior removal of moisture from the filter, with the risk of crack formation.
Further advantageous developments of the method according to the invention are described in the dependent claims.
An alternative apparatus for implementing the method according to the invention is given in the apparatus claim. The filter cake is exposed to treatment fluid under a hood, the rest of the area being exposed to a different gas.
In processes of pressure filtration or enclosed vacuum-filtration, the entire process area, or more advantageously, within the process area, only the treatment area separated by an internal additional enclosure above the filter cake, is exposed to the treatment fluid. According to a further development, the gas exchange between the two pressure-chambers is minimized, without having to provide expensive seals. The low thermal conductivity of the gas contained in the first pressure-chamber provides thermal insulation of the filter cake from the external environment of the apparatus, thereby increasing its efficiency.
The invention is described below, by reference to an example, referring to the drawing. The drawing shows:
FIG. 1 a cross-section through a first apparatus for the implementation of the method, and
FIG. 2 a cross-section through a second apparatus for the implementation of the method.